Improvement upon the carrier precipitation of plutonium ions from nitric acid solutions



Patented Dec. 23, 1958 IMPROVEMENT UPON THE CARRIER PRECIPITA- TION OF PLUTONIUM IDNS FROM NITRIC ACID SOLUTIUNS Ralph A. James and Stanley G. Thompson, Chicago, 111., assignors to the United States of America as represented by the United States Atomic Energy Commission N Drawing. Application March 14, 1946 Serial No. 654,505

2 Claims. (Cl. 23-14.5)

This invention relates to an improvement in the process of preparing a pure radioactive element. More particularly it pertains to a novel method of recovering element 94 from neutron-irradiated uranium dissolved in a suitable acid wherein the element 94 contained in the resulting solution is subjected to reduction and removed from said solution by means of a suitable carrier prior to the separation of any impurities that are normally associated therewith.

It is known that when uranium is subjected to neutron bombardment there is formed in small quantities a new element having an atomic weight of 239 and atomic number of 93, known as neptunium (symbol Np). This new element by radioactive decay is transformed through a half-life of 2.3 days to a further new element having an atomic weight of 239 and atomic number 94, known as plutonium (symbol Pu). Other isotopes of plutonium may also be formed. In addition certain other elements are formed as a result of fission of the uranium 235 nucleus, such new elements being referred to as fission fragments or, including radioactive decay products thereof, as fission products. The fission fragments include two general element groups, a light fission fragment such as Br, Kr, Rb, Sr, Y, Zr, Cb, Mo, element 43, Ru, Rh and a heavy fission fragment such as Sb, Te, I, Xe, Cs, Ba, La, Ce, Pr, and Nd. The fission fragments are usually present in a form undergoing radioactive decay and many of the fission fragments form other short-lived products. As a result the radioactivity of the mass of uranium remains at a high and very dangerous level for some time following high-density neutron bombardment. It is particularly desirable to separate the plutonium from the radioactive fission fragments and fission products thereby removing from the mass subjected to neutron bombardment the radioactive materials and particularly the light elements such as light metals having very short halflives and consequently high radio-activities.

It is an object of the present invention to provide an improved method of recovering plutonium from its solution.

It is a further object of the present invention to provide a simple and efiicient method of recovering plutonium in quantitive amounts with a minimum of loss.

Further objects of the present invention will be apparent from the following description.

Following the bombardment of uranium with neutrons to produce fission products and/ or new elements as discussed above, the usual procedure is to dissolve the entire mass, after it has aged to a point where a substantial portion of the U has decayed to Pu in an aqueous acid solution such as nitric acid. This solution then contains the uranium, plutonium usually in the tetravalent state, and all of the other products of the neutron bombardment including radio-active fission products.

The recovery of plutonium from such solutions is generally effected by the formation of a suitable insoluble compound in said solution which is capable of carrying plutonium in a valence state not greater than +4. The

carrier precipitate and the plutonium contained therein are separated from the solution by filtering or centrifuging and then dissolved and the plutonium is oxidized to PuO in which state of oxidation it does not become associated with said carrier. Under these conditions, the plutonium remains in solution and the fission products are removed when the carrier is formed and precipitated. Thereafter the dissolved plutonium is reduced to a valence not greater than +4 in which state it is removable by the aforesaid carrier and separated from the solution in the form of a carrier precipitate which may again be dissolved and the plutonium then may be purified further if considered necessary or desirable by repeating the above cycle.

Despite the fact that this process was found to remove plutonium effectively in small sc g le operation, it has been frequently found that objectionable quantities of plutonium remain in the initial solution where large scale operation is conducted, and recoveries in such processes have been irregular in many cases.

On occasion it has been observed that from 10% to as much as 60% of the plutonium present Would not be carried. In view of the fact that the plutonium may often be present in extremely small quantities of the order of 10-30 mg./l., it is apparent that the procedure employed to recover that element must be highly efiicient in order to be at all practicable and therefore, any procedure wherein as much as 60% of the plutonium is not carried would obviously be unsuitable.

In accordance with the present invention it has been discovered that the plutonium may be readily recovered from a solution containing plutonium in a lower valence state (not above 4) in uniformly high yield if the solution of neutron-irradiated uranium containing plutonium in ionic state is provided with nitrite ions prior to absorption or carrying, or extraction by hexone, ether or other organic solvents of plutonium therefrom.

It was originally believed that the reason for incomplete carrying of plutonium was that some of the plutonium was in the higher oxidation state of +6. However, it has been found that even when no +6 plutonium is present, the carrying of plutonium was often incomplete. This was particularly the case when the solution resulted from dissolving uranium metal in nitric acid and when attempt was made to remove plutonium in the +4 state. When there is no plutonium present in the nitric acid solution having a valence above +4, obviously there is no need for reduction of plutonium. Nevertheless it has been found that reagents such as nitrite do improve carrying of plutonium in such solutions.

A possible reason for this phenomenon is that a small amount of hydrazine, N H or similar powerful reducing or complexing agent is formed when uranium metal is dissolved in nitric acid. These agents when present interfere with the carrying of plutonium either by complexing the plutonium ions, or by reducing it to the trivalent state.

It is believed that the nitrite supplied to or generated in the solution functions by destroying the hydrazine or similar agent present. In addition to being capable of destroying hydrazine or other similar reducing agents, it has been found that the presence of nitrite ions tends to favor the existence of plutonium ions in the tetravalent state. This discovery is indeed most interesting inasmuch as the tetravalent plutonium normally undergoes at least partial disproportionation into the triand hexavalent states in dilute solutions of hydrochloric acid, nitric acid, or perchloric acid. Thus, in solutions containing plu tonium ions in disproportionation equilibrium nitrite will upset this equilibrium and convert plutonium which is in a valence state other than +4 to the tetravalent state in which form it is very stable. Such a phenomenon is exceedingly helpful in the removal of plutonium from solution by the use of carriers .such as those of the bismuthphosphate-type which function satisfactorily only when the plutonium is in the tetravalent state. I

" The process may be most ffe theplut on'ium solution contains an agent capable of stabilizin g' tetravalent plutonium such as sulphate, phosphate, tartrate, citrate orfi'uo'ride ionsi "These ions preferably should be prsent'in concehtrationsatleast as highlas the plutonium committee arid "genera ly in amounts several times the plutonium content of: the solution.

The nitrite ions may be' provided'either by adding a suitable nitrite salt orby prev nting escape of NO which is produced 'duringreactioii-of the HNOiwithhr'iiium metal, or the nitrite ions may be'p'rovided' by adding a reducingagent to th'nitric'acid solution which'reduces nitric 'ac'idto nitrite ion.

A number of different substances have been" found suitable as reducing agents to improve car'ryiiig 'of 'pluto nium by providing nitrite ion from the reduction of nitric acid. Among suchreducing agents are hydrox'ylarnine, sulphur dioxide, phenylhyd'r'azine, hydrogen peroxide, formic acid together with ferrous ion as a catalyst, ferrous ion alone, and urano-us ion (U "Other'r'educingag'ents having an oxidation-reduction potential above 1.1'volts as shown, for instance, in the 'Latim e'r and Hildebrand table of oxidation-reduction potentials also may be used.

In carrying out the present -invention, a nitrite'salt or a suitable reducing agentjis added to the'nitric acid solu: tion containing plutonium, fission products, and u'r'anyl nitrate and the resulting mixture is heated tofa temperature of between about 50 and 100C. for a period of between approximately one and two hour'sdepending chiefly on the particular agent employed; j In general, however, temperatures of about 75 C. will be found satisfactory. The time required to convert'the plutonium to a carriable state will dependinpart on the concentration of residual. nitric acid, the reducing agent used, and the temperature at which treatment is carried out. Generally, with any of the agents mentioned above; conversion of thepluto'nium to a carriable' state .will be found to be complete within about two hours. "The solution can then be treated with a suitable carrier, such as for example, bismuth phosphate, lanthanum fluoride, in accordance with the general procedure set forth above.

The term carrier as used herein and in' the appended claims signifies a substantially"insoluble, solid, finely divided compound capable of ionizing to yield at least one inorganic cation and at least one anion which constitutes an ionic component of a compound which is-not substantially more soluble than said finely divided com pound and which contains the ion (plutonium ion in the present instance) to be carried. v

A large number of carriers is available for use in the process of our present invention. Numerous individual carriers have previously been used' in extraction and decontamination processes, and combinations of these carriers are suitable for use in our present process. The following are representative examples of useful carriers.

The following examples illustrate the present invention. All parts are by'weight.

EXAMPLE 1 of 93% sulfuric "acid. To this"solution"there were next added 10 lbs. of water,"5" lbs of 60% nitric acid, and 613 lbs. of 75% phosphoric acid followed by lbs. of water. The solution was then heated to 75 C., and 83.3 lbs. of 24% bismuth nitrate solution was addeduniforrnly during a periodoffthirtyminutes followed by 5 lbs of nitri'ci 'a'cid. The solution'was then digested for two hours at 75" C., cooled to 35 ciyarter' which thep'recipitate was removed by'means of a centrifuge; 98% of' tlrejplutoniumpresent in the original solution was recovered "fro'rr'i" said precipitate.

' The above d scription ,isfllustrzitive of the details of the procedure used in practicing theprelsejnt invention.

.O b viously such procedure may be 'siibstantially 'varied without departing from the invention.

v he following table shows the" effect ofjthe'a dit'ion of formicacid'to a'nitric acid solution of ammo-irradiated uranium ontheperc'enta'g'e of plutonium recovery. The-"procedure used was similar tothat'given above.

Table I Percent Conditions of Extractions plutonium V in waste solution 0101 M U added 3- to precipitator uniformly during 30 minutes-no formic acid added 2 0.05 M formic acid added to precipitator uniformly during 130 i 'l 0.05 M formic acid added to the dilution water N0 formic acid added N o formic acid added and one-half the usual amount of bismuthion used. 0.5 M formic acid added uniformly during 40 minutes. N o formicacid added; bismuth ion added uniformly (1 mg 120 minutes As can be seen from the above table the plutonium waste losses when no formic acid is added may amount to as'much as 21.2 percent. On the other handwhen formic acid is used the plutonium waste losses can be reduced to as little as 1.2 percent. The following experiments were carried out 'using potassium nitrite in place of formic acid:

containing hydrazine in a concentration L' fQOlM, nitric p acid in .a concentration-of 5%, l0% sulfuric-acid, and

EXAMPLE II A O'fl Msolution of'potassiun'i 'riitrite was added with the dilutionwater to a nitric acid solution of neutronirradiated uranium. The solution was agitated for a period of thirty minutes at 40 C. and heated to 75 C. prior to treatment 'with a carrier in the mannerdes'cribed above. The average plutonium loss for 10 runs using this procedure was 1.6%. v p

Controleiqperiments seem toicle'arly indicate that hydrazine is an interfering substance. A Thus when hydrazine is added to solution of 'neutronirradiated uraniurnjin nitric acid the plutonium loss is substantial but the loss is greatlyi'reduced-when. formic acid or'a suitable 5 nitriteisadded 'asfshown 'in'the following example.

EXAMPLE III A uranium solution containing plutonium was made up 1.5 "111 1 01: formic acid per 'mol of nitric acid.

this solution was treated in accordance with the foregoing description there resulted a plutonium loss of only 0.6%. A similar run was carried out in the absence of formic acid with a loss in plutonium of 95%.

In order to show the efiect of potassium nitrite on the recoverability of plutonium from nitric acid solutions the following experiment was carried out.

EXAMPLE IV Product Not Carried Sample 0.01M NzH4, 0.01M N2H -lpercent 0.025M

KN 0 percent Starting solution 1 100. 0

15 minute digestion 30 minute digestion-.." 45 minute digestlon- 60 minute digestion 90 minute digestlon 120 minute digestlom.--

mew-mamas NE PFWS". ooooooo The following example illustrates that the process is applicable to large scale operation using sodium nitrite as the active agent.

EXAMPLE V Neutron-irradiated uranium metal containing about 250 grams of plutonium per ton of uranium was dissolved by addition of 60% nitric acid in the proportion of 5.5 mols of HNO per mole of metal and maintaining the temperature at about 110-115 C. The solution obtained was cooled and diluted to about 20% uranyl nitrate hexahydrate. Sufficient sulphuric acid was added to establish a concentration of 0.204 pound of H 50 per pound of uranyl nitrate hexahydrate. The temperature of the solution was then adjusted to 40 C. and suliicient 25% NaNO aqueous solution was added to make the solution 0.1 molar in NaNO and the solution was heated to C. for one hour.

Thereafter BiONO was added to the solution in amount suflicient to give a bismuth concentration of 2.5 grams per liter of solution and then 75% H PO was added over a 2-hour period to give a solution about 0.6 molar in H PO The mixture was digested for one hour at 75 C., cooled to 35 C., centrifuged, and the precipitate was removed. Substantially all of the plutonium was removed with the precipitate while most of the uranium was left in solution.

Although the invention has been described with ref erence to removal of plutonium in tetravalent state with carriers, it als'o'may be applied to removal of plutonium from solutions containing ionic plutonium in tetravalent state with other agents including absorbents for plutonium, such as amberlite resins or titanium dioxide, or solvents containing ketone or other groups, such as hexone, methyl ethyl ketone, diethyl ether, Cellosolve, etc.

What is claimed is:

1. A process for improving the carrying of plutonium on an insoluble carrier precipitate from a hydrazinecontaining nitric acid solution of plutonium in a maximum valence state of +4 derived by dissolving neutronbombarded uranium metal in nitric acid which comprises adding to said solution sufiicient potassium nitrite to make the solution about 0.1 molar and to destroy the hydrazine prior to contacting the solution with said carrier.

2. In a process of recovering plutonium values by carrier precipitation from a nitric acid solution containing said plutonium values in the tetravalent state and hydrazine in a concentration of about 0.01 M, the step of adding potassium nitrite in a quantity to yield a concentration in the solution of about 0.025 M prior to incorporation of the carrier.

References Cited in the file of this patent UNITED STATES PATENTS 2,785,951 Thompson et a1. Mar. 19, 1957 

1. A PROCESS FOR IMPROVING THE CARRYING OF PLUTONIUM ON AN INSOLUBLE CARRIER PRECIPITATE FROM A HYDRAZINECONTAINING NITRIC ACID SOLUTION OF PLUTONIUM IN A MAXIMUM VALENCE STATE OF +4 DERIVED BY DISSOLVING NEUTRONBOMBARDED URANIUM METAL IN NITRIC ACID WHICH COMPRISES ADDING TO SAID SOLUTION SUFFICIENT POTASSIUM NITRITE TO MAKE THE SOLUTION ABOUT 0.1 MOLAR AND TO DESTROY THE HYDRAZINE PRIOR TO CONTACTING THE SOLUTION WITH SAID CARRIER. 